Switch to Cone for shipMarker

[Ian Mayo]

When we integrate 3d vessel models we'll need to support heading.

As a stepping stone to this, there would be merit in switching the MarineTracksLayer from PointPlacemark to Cone - with the Cone oriented along the current vessel heading (well an interpolated heading between the data points either side).

So, as we move forward in time we'll see the cone pointing in the direction of travel - like a very-simple vessel model.

[Lance Wellspring]

Unfortunately this is not as simple with PointPlacemark as it was with BasicMarker. There is no 'set shape' function for PointPlacemark, so I would have to implement the openGL drawing of it myself (or find usable existing code for it).

Right now PointPlacemark simply does

gl.glBegin(GL.GL_POINTS);
gl.glVertex3d(this.screenPoint.x, this.screenPoint.y, 0);
gl.glEnd();

after setting scale, so its literally just a simple point, oriented to always face the screen. It will take some investigation to determine whether this enhancement will be easy or not.